1. Field of the Invention
The present invention relates to an improved method of manufacturing diesters of polytetramethylene ethers. More specifically but not by way of limitation, the invention relates to the polymerization of tetrahydrofuran (THF) with or without a second cyclic ether comonomer utilizing a solid acid catalyst and carboxylic acid with carboxylic acid anhydride as molecular weight control agents wherein setting the pressure of the reactor while simultaneously achieving removal of the heat of reaction by operating under solvent/monomer evaporative reaction conditions effectively controls the reaction temperature.
2. Description of the Related Art
Polytetramethylene ether glycol (PTMEG) is a commodity in the chemical industry which is widely used to form segmented copolymers with poly-functional urethanes and polyesters. PTMEG is commercially prepared by reacting tetrahydrofuran (THF) with fluorosulfonic acid and then quenching the product with water. While this process has proved to be quite satisfactory, it is not as efficient as desired because the acid cannot be recovered and reused. Moreover, disposal of the spent acid is a problem because of its toxicity and corrosiveness.
U.S. Pat. No. 4,120,903, discloses the polymerization of THF using a polymer containing alpha-fluorosulfonic acid groups as a catalyst and water or 1,4-butanediol as a chain terminator. The nature of the catalyst permits its reuse, thereby eliminating the disposal problems and the catalyst's lack of solubility in the reaction mass makes it easy to separate the (catalyst from the product at the end of the polymerization reaction. This very low solubility also minimizes loss of catalyst as the reaction proceeds. However, this process produces a polytetramethylene ether glycol having a molecular weight of 10,000 or more, while the commercial products generally have molecular weights of less than 4,000 with the majority of commercial products having a number average molecular weight from 650 to 3,000.
U.S. Pat. No. 4,163,115 discloses the polymerization of THF and/or THF with comonomers to polytetramethylene ether diester using a fluorinated resin catalyst containing sulfonic acid groups, in which the molecular weight is regulated by addition of an acylium ion precursor to the reaction medium. The patent discloses the use of acetic anhydride and acetic acid in combination with the solid acid catalyst. The polymeric product is isolated by stripping off the unreacted THF and acetic acid/acetic anhydride for recycle. The product isolated is the diacetate of polymerized tetrahydrofuran (PTMEA) which must be converted to the corresponding dihydroxy product, polytetramethylene ether glycol (PTMEG), to find application as a raw material in most urethane end use applications.